Approaching the planet Atrios, at war with it's close neighbour Zeos, the Tardis is attacked by a nuclear warhead. On Atrios opposition is growing to The Marshall military leadership of the war, lead by Princess Astra and Surgeon Merak. Under the Marshall's orders Astra is led astray by her guard and shut in a highly radioactive area. Arriving on Atrios, The Doctor breaks into the sealed K-block , where he & Romana see the missing princess Astra, but they are captured by the Marshall and accused of being spies alongside surgeon Merak. Astra is abducted by a black robed & masked figure. The Doctor, Romana & Merak are freed by K-9 but when they flee they discover the Tardis gone.

Bit of a setup episode with little indication of where the sixth and final segment of the Key To Time is. But the situation, two planets at war with each other, is clearly explained and a hint is dropped that Zeos isn't where it should be.

Two notable landmarks are reached this episode. Firstly it's Doctor Who's 500th episode, and secondly it's Tom Baker's 119th episode bringing him level with Patrick Troughton but still behind Jon Pertwee on 128 and William Hartnell on 134.

The male actor in the public information film that opens the show is Ian Liston, better known amongst sci fi fans for playing Wes Janson, Rogue 3's/Wedge's Snowspeeder gunner in The Empire Strikes Back. Meanwhile the credited guard in this episode is John Cannon who was Elgin in The Hand of Fear and later scheduled to play an executioner in Shada